Sex Myths: STDs Are Transmitted Every Time You Have Sex

The Truth About STD Transmission

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There are a lot of sex myths that revolve around how to tell whether or not you have a cheating partner. One particularly poisonous idea is the myth that if your long-term partner is suddenly diagnosed with a new STD infection, it must mean they're cheating. After all, if you were the source of the STD, your partner would have been infected a long time ago. Right?

Wrong! There is a simple, incorrect assumption lurking behind this sex myth.

That assumption is that if you have an STD, it will be transmitted every time you have sex. But that isn't actually true. In fact, there are a number of factors that can affect the likelihood of any particular STD being spread. The risk can be quite different depending on the circumstances surrounding any encounter where you are having sex.

Factors That Can Increase or Decrease the Likelihood of STD Transmission

There are a number of factors might affect whether or not you will pass on an infection during a sexual encounter. These include:

  • the type of infection.
  • the amount of virus/bacteria in your blood or in your genital secretions. This is related to the severity of your infection. 
  • whether you or your partner have any open sores or other wounds on your genitals. Similarly, whether you have broken skin in other areas where you'll have skin-to-skin contact during sex.
  • the presence of other STDs in either you or your partner.
  • how much time you spend having sex.
  • whether or not you have safe sex, and whether you have it correctly. 
  • whether or not you use sexual lubricants.
  • the type of lubricants you use.
  • the gender of the infected partner.
  • the gender of the uninfected partner.
  • the types of sex you are having (e.g. anal, vaginal, oral.)
  • the overall health of you and your partner, in particular, your immune system.

The absolute risk of an STD being transmitted during any given sexual encounter varies a lot. Just because your partner has only recently been diagnosed with an STD when the two of you have been together for a long time, it does not necessarily mean that they have been cheating on you.

In such a circumstance, there are actually several possible alternate explanations. These can be difficult to choose between. That's particularly true if you weren't both screened for STDs before getting into the relationship.

For example, imagine if you turn out to be asymptomatically infected with the same STD but weren't screened before getting together. In that case, there is a possibility that you were infected the whole time. You may have only recently transmitted the disease to your partner. They may only have recently developed symptoms. You both could have been infected since before your relationship started. You may simply not have recognized the symptoms until just now.

One Final Caveat

STDs aren't necessarily transmitted every time you have sex. However, that doesn't mean you can assume they won't be transmitted during any particular time you are having sex.

Your health isn't something you want to trust to luck. Therefore it's is your responsibility to do what you can to minimize your risk. Practicing safe sex is a lot more reliable than relying on probability or prayer. That's particularly true when safe sex is combined with appropriate testing.


Baeten, J.M. et al. "Genital HIV-1 RNA Predicts Risk of Heterosexual HIV-1 Transmission" Sci Transl Med. 6 April 2011 3:77ra29.

Boily M.C. et al. "Heterosexual Risk of HIV-1 Infection Per Sexual Act: Systematic Review and Meta-Analysis of Observational Studies." Lancet Infect Dis. 2009 Feb;9(2):118-29.

Fleming D.T. et al."Herpes Virus Type 2 Infection and Genital Symptoms in Primary Care Patients." Sex Transm Dis. 2006 Jul;33(7):416-21.

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